The present invention relates to an electrostatic puncture preventing element, and is mainly directed to a protective element for preventing electrostatic puncture in a semiconductor integrated circuit.
As a technique for preventing electrostatic puncture of a semiconductor integrated circuit (IC), there has hitherto been known a technique in which, as illustrated in FIG. 3, a resistor R is connected on the input side and in series with an integrated circuit 7, and the waveform of a surge pulse is smoothed by the time constant between a stray capacitance C and the resistance R, so as to prevent an abrupt surge pulse from entering the integrated circuit.
As another technique, there has been one in which, as illustrated in FIG. 4, a diode D adapted to break down by a surge pulse, is connected on the input side of and in parallel with an integrated circuit 7, to thereby absorb the surge pulse.
Regarding the former technique, it has been revealed that in constructing the resistor of a P-type region formed in an N-type semiconductor substrate, when a forward surge pulse is applied to the PN junction between the substrate and the P-type region, that is, a positive potential is applied to the resistor, the resistor does not puncture, whereas when a large surge pulse (-) is applied in the reverse direction, the resistor itself is punctured if the pulse is of a voltage exceeding the breakdown voltage of the PN junction.
Also, regarding the latter, frequently when reverse voltages exceeding the breakdown voltage of the junction of the diode are applied, the diode itself is punctured.
For these reasons, the prior-art protective elements have not been sufficiently protective. For example, they have been punctured by merely being applied with a surge pulse of a reverse voltage of one hundred and several tens of volts from an external terminal.
Thus, they have been punctured when applied with a reverse surge pulse of a voltage exceeding 100-130 V, and it has heretofore been unrealizable to make an IC having a breakdown voltage of over 100 V.
Consequently, ICs have easily been permanently punctured even by the slight electrostatic energy charged in the human body.
Particularly, when employing an IC in an electric circuit which generates a high voltage as in a television circuit, an automobile, etc., a surge pulse of 250V, 500V or higher is sometimes impressed on the circuit, and the puncture strength of the IC has come into question.
The present inventor has investigated a means to prevent the puncture preventing element and the circuit from been punctured even when a surge pulse of several hundred volts is applied from a signal input terminal.
It has been revealed from this investigation that where a forward surge pulse is applied to the PN junction of a semiconductor, the junction is not punctured even at a pulse voltage of several hundred volts, whereas where a reverse surge pulse is applied, it is punctured at a low voltage of 100-130 V.
The reason for this is not definite, at present, but the phenomenon is considered to be attributable to the fact that when a high reverse bias voltage is suddenly applied, currents are locally concentrated when a high voltage is applied on both sides of a depletion layer before the occurrence of breakdown.
The inventor of the present application has therefore devised connecting an element having transistor structure in parallel with the input terminal of the circuit to be protected in an IC substrate, so as to spuriously effect forward operation, whereby, when a surge pulse in either the forward or reverse direction is applied, the element portion is caused to operate as a transistor, in order to absorb the surge pulse.
The present invention has been developed as the result of the above investigation, and has as an object the provision of a puncture preventing element which is difficult to puncture for surge pulses in both the forward and reverse directions.